1. Field of the Invention
The present invention relates to a method of regenerating an exhaust gas post-processing apparatus that is provided in an exhaust gas path of a diesel engine installed in an engine-driven generator and that removes particulate matter (hereinafter referred to as “PM”) from the exhaust gas, and a regeneration apparatus for such an exhaust gas post-processing apparatus.
2. Description of the Related Art
Engine-driven generators that comprise a generator body and an engine for driving this generator body have been used to secure power supply in outdoor workplaces such as civil engineering and construction sites and outdoor event sites, where no commercial power supply can be secured, and have also been used as emergency power supplies for power outages or disasters.
In such engine-driven generators, a diesel engine is typically used as an engine for driving the generator body. However, because of their structure, diesel engines emit a larger amount of PM together with exhaust gas upon combustion, as compared with gasoline engines.
Since PM causes air pollution and health hazards, regulation values (mass per unit output [g/kWh]) for PM emitted from a diesel engine are determined by emission gas regulation.
To adapt to this emission regulation, gas post-processing apparatuses with a diesel particulate filter (hereinafter referred to as a “DPF”) are installed in exhaust gas paths of diesel engines in order to reduce the emission amount of PM.
In this exhaust gas post-processing apparatus, the built-in DPF collects PM in exhaust gas in order to reduce the emission amount of PM. Therefore, with continuous use, the deposition of PM with respect to the DPF proceeds and eventually clogs the DPF.
Increase in the resistance for the exhaust gas due to the clogging of the DPF results in lowering the engine output and deteriorating fuel efficiency. Thus, it requires a process to regenerate the DPF by removing PM deposited in the filter medium.
As an exemplary scheme for regenerating a DPF, an exhaust gas post-processing apparatus called as a “continuous regeneration type” exhaust gas post-processing apparatus has been suggested, in which a diesel oxidation catalyst (hereinafter referred to as a “DOC”) is provided on its inlet side and a DPF is accommodated on the downstream side so that the DPF is continuously regenerated due to the catalyst effect of the DOC.
This continuous regeneration type exhaust gas post-processing apparatus is for continuously burning and removing PM by means of the heat of the exhaust gas while the engine is running, in which apparatus NO2 is generated by the action of DOC when heated to its activating temperature or more by the exhaust gas from the operating engine, and the NO2 can be used as an oxidizer in burning of PM deposited with respect to the DPF so as to regenerate the DPF at a temperature lower than that at which the PM burns by itself with oxygen.
However, even in the above continuous regeneration type exhaust gas post-processing apparatus, when the engine runs for a long time with the temperature of the exhaust gas below the activating temperature of DOC, such as when the engine runs under a light load for a long time, NO2 is not generated and the PM cannot be burnt. Consequently, the deposition of PM with respect to the DPF proceeds.
Once the engine transits to heavy-load operation after PM is deposited with respect to the DPF beyond a certain amount and the resistance for the exhaust gas is thus elevated, the elevated resistance for the exhaust gas increases the temperature of the exhaust gas to higher than that in normal heavy load operation. As a result, a large amount of PM deposited in the DPF starts to burn by themselves so as to emit high heat, which causes cracks or melts in the DPF.
Therefore, a forced regeneration scheme is also used in combination with the continuous regeneration type exhaust gas post-processing apparatus, in which scheme, when the deposition amount of PM with respect to DPF becomes or exceeds a predetermined amount, a temperature of the exhaust gas is increased by additionally injecting fuel or delaying the injection timing and thus a temperature of the DOC in the exhaust gas post-processing apparatus is increased, so that PM deposited in DPF is forcibly burnt with NO2 as an oxidizer (Japanese Patent LOPI No. 2001-280118).
The followings should be noted. In the forced regeneration scheme according to the above-mentioned Japanese Patent LOPI No. 2001-280118, the forced regeneration is performed in a state such as a running state of an automobile in which a load placed on the engine varies (hereinafter referred to as a “variable load type forced regeneration”). If this variable load type forced regeneration starts when the engine runs under a light load and the exhaust gas has a low temperature, it may take a long time to completely regenerate the DPF. In addition, when a load on the engine varies, the amount of fuel to be additionally injected is fluctuated, or when the engine temporarily runs under a heavy load, additional injection of the fuel is repeatedly stopped, as a result, a temperature of the oxidation catalyst becomes unstable and cannot be kept in its activating temperature or more, thus failing to continuously and stably generate NO2 and completely burn the PM. Moreover, if an operator stops the engine in the course of the execution of the forced regeneration, regenerating the DPF is suspended. In this case, the PM deposited in the DPF may be removed insufficiently and the deposition of PM may further proceed. Then, if the large amount of the deposited PM burns intensely by itself, the DPF body and the filter medium might be damaged. In light of these risks, the applicant of the present application filed a patent application (PCT/JP2014/073178) regarding a method of regenerating an exhaust gas post-processing apparatus installed in an engine-driven compressor. In this method, an operator starts forced regeneration by operating a switch or the like on the basis of, for example lighting of a warning lamp which indicates that at least a predetermined amount of PM is deposited with respect to a DPF so that the forced regeneration can be performed while a constant load is placed on the engine, namely, the temperature of the DPF is kept stable (hereinafter, this type of forced regeneration is referred to as a “constant load type forced regeneration”).
The above-described exhaust gas post-processing technology has been developed primarily for diesel engines in automobiles. However, PM emission restrictions should also be imposed on engine-driven generators and other industrial machines equipped with a diesel engine. In present, requests for providing environmentally friendly products are setting up demands for equipping even engine-driven generators with an exhaust gas post-processing apparatus.
However, when the above-described continuous regeneration type exhaust gas post-processing apparatus is installed in an engine-driven generator without any modification, the activity performance of the DOC deteriorates at a relatively early stage. This may inhibit PM deposited in the DPF from being burned and removed sufficiently during forced regeneration. This problem has not been found when a continuous regeneration type exhaust gas post-processing apparatus is installed in an automobile.
Suppose that the activity performance of the DOC deteriorates and PM deposited in the DPF cannot be removed completely in the forced regeneration and some PM is left therein, the DPF may be clogged at shorter time intervals, accordingly, fuel should be additionally injected more frequently. As a result, the fuel efficiency might deteriorate and a larger amount of unburned fuel is generated as a larger amount of the fuel is injected. Consequently, an oil dilution phenomenon, i.e., mixing the engine oil with the unburned fuel, is likely to occur, thus the engine oil is prone to deteriorate at an earlier stage and damage the engine accordingly.
An investigation of the cause of the deterioration at an earlier stage of the activity performance of the DOC in an exhaust gas post-processing apparatus installed in an engine-driven generator revealed that PM was deposited with respect to not only the DPF but the DOC in the exhaust gas post-processing apparatus installed in an engine-driven generator.
Another investigation of the cause of deposition of PM with respect to the DOC concluded that the use mode of a diesel engine in an engine-driven generator is different from that of a diesel engine in an automobile so that a lower-limit temperature of the exhaust gas was lower in the engine in the engine-driven generator compared with the diesel engine in an automobile.
More specifically, a heavier load on an engine makes the burning temperature inside the combustion chamber and the temperature of the exhaust gas higher, while a lighter load on the engine makes both the temperatures lower. When the driver stops an automobile over a long time or parks it, he/she stops its engine. Thus, the engine rarely runs under no load (idling operation) over a long time. In contrast, even when the engine runs under a light load, such as when the automobile runs at a low speed, a certain load is placed on the automobile, because the engine is rotating wheels which bear the weight of the automobile, in which case the exhaust gas is maintained at some high temperatures.
While an engine-driven generator is consuming no electricity, such as while an electrical device connected thereto is stopping its operation, an engine-driven generator may be still operating at idle without stopping its operation and ready for a following startup of the electrical device. As long as at least an electrical device is connected to an engine-driven generator, the engine-driven generator may continuously operate without stopping the engine even if this electrical device consumes relatively low electricity. In short, an engine-driven generator may continuously operate over a relatively long time while consuming no or little electricity.
The torque required to rotate a generator body increases along with its output power, and when no power is output, almost no load is placed on the engine. Thus, an engine in an engine-driven generator may operate at a lighter load than an engine in an automobile. In other words, the engine in the engine-driven generator may create exhaust gas whose typical temperature is lower than that from the engine in the automobile.
To burn PM deposited in a DOC, it is necessary to increase a typical temperature inside an area in the vicinity of the DOC inlet to preferably about 350° C. or more, more preferably 400° C. or more, although this temperature is dependent on the performance and the like of the DOC.
The DOC inlet temperature in the engine-driven generator which had a deteriorated activity performance was measured. According to this result, the measured temperature did not reach that required to burn PM during both a normal operation in which no fuel is additionally injected and forced regeneration in which the fuel is additionally injected to increase the temperature of exhaust gas. This result thus reveals that the DOC inlet temperature in the engine-driven generator does not have a sufficiently high temperature as long as the generator generates low electricity, namely, operates under a light load.
It is confirmed that an exhaust gas post-processing apparatus in an engine-driven generator, when increasing the temperature of the exhaust gas by additionally injecting fuel, fails to remove PM deposited in the DOC before regenerating the DPF. Moreover, it was observed that when the additional injection of the fuel increases the amount of unburned fuel in the exhaust gas, the unburned fuel adheres to the PM deposited in the DOC and the deposition of PM is further promoted, which is a vicious cycle.
As a result, the activity performance of the DOC deteriorates and thus PM deposited in the DPF also cannot be removed, which is a more vicious cycle.
To overcome the above problem, that is, to break the above vicious cycle, the inventor of the present invention conceived of a novel regeneration method that involves two processes. More specifically, in the first process, the temperature of exhaust gas is increased without an additional injection of fuel to burn and remove PM deposited in a DOC for regenerating the DOC. Then, in the second process, the fuel is additionally injected for regenerating a DPF.
To increase the temperature of the exhaust gas without additionally injecting the fuel upon the regeneration of the DOC, the inventor attempted to restrict intake gas to an engine.
According to Table 1 and FIG. 5, however, when an engine runs under a light load, simply restricting the intake gas may fail to increase the DOC inlet temperature to a temperature at which PM deposited in the DOC burns (350° C. or more, more preferably 400° C. or more). This result teaches that another approach is needed in order to accomplish a novel challenge of regenerating a DOC.
TABLE 1ELECTRICITY OF GENERATOR BODYVS DOC INLET TEMPERATUREGENERATEDDOC INLET TEMPERATURE (° C.)ELECTRICITYINTAKE GASIN NORMAL(kW)RESTRICTEDOPERATION4260161628817983222001037723812406254
The present invention has been made to solve the above-described problems that has newly arisen when a continuous regeneration type exhaust gas post-processing apparatus equipped with a DOC is installed not in an automobile but in an engine-driven generator. An object of the present invention is to provide a method of regenerating an exhaust gas post-processing apparatus in an engine-driven generator in which the method allows during the forced regeneration to ensure the removal of PM deposited in the DOC provided in the exhaust gas post-processing apparatus with a relatively simple method and apparatus configuration, thereby surely removing PM deposited in the DPF and regenerating the DPF, accordingly, the method can prevent too frequent regeneration due to poor regeneration of the DPF, as well as accompanying fuel efficiency deterioration, oil dilution, and the like. Furthermore, an object of the present invention is also to provide a regeneration apparatus for performing the method.